MFA for a custom web application on a user's smartphone is no longer working.

Comprehensive and Detailed Explanation From Exact Extract:

Multi-Factor Authentication (MFA) apps, especially time-based one-time password (TOTP) apps (e.g., Google Authenticator, Authy), rely on accurate time synchronization between the device and the authentication server. If the user recently traveled internationally, the device may have incorrect date/time settings due to time zone changes or failed synchronization, leading to MFA failure.

The most logical and non-intrusive first step is to verify and correct the date and time settings. This aligns with basic troubleshooting principles—start with the simplest and most likely cause before taking more drastic action.